Satellite imaging is an important tool used by governments and commercial entities to gather information about terrestrial-based activities and features. The information sought may pertain to activities such as terrain mapping, weather forecasting, scientific research, intelligence gathering, navigation, communications, agriculture, ecology, pollution monitoring, urban planning, water resources management, mineral exploration, and the like. In terrain mapping, the satellite takes real-time images of the Earth's surface, and objects located thereon, and can accurately represent the present state of the terrain as well as identify any changes, e.g., new structures and geological formations. For the intelligence gathering function, satellites are invaluable for monitoring areas of strategic importance and provide real-time feedback for military and political planning. Governments need to know activities associated with foreign installations and movement of people and equipment.
Satellites are generally known or classified as passive or active type systems. A passive satellite reflects or scatters incident radiation from one Earth station to another, i.e. radio signals are transmitted by reflection. A passive system is designed to look down from Earth orbit and receive incident radiation, or take a digital photograph, which is representative of an image of the terrain or specific objects located on the Earth's surface. Passive systems can provide good resolution images, but generally cannot see through cloud cover or other obstacles and are limited to operation only during daylight hours.
Active satellite systems use more advanced technologies than passive systems. An active satellite has transmitting equipment aboard, such as a transponder, power supplies, and signal processing equipment. The transponder receives a signal from Earth and retransmits the signal back to Earth. The transponder and signal processing equipment may perform operations such as amplification, filtering, regeneration, frequency translation, link switching, and other signal processing before re-transmission back to Earth. The re-transmission may occur immediately or after a short delay. Most active satellites have more than one transponder and antenna so that they can relay several radio waves or signals at the same time.
One area of interest for satellites is known as active imaging. Active imaging satellites are typically placed in geostationary, low earth orbit (LEO), e.g., about 400-1,000 km in altitude. The satellite paints the area of interest with high frequency radio signals, which are reflected back to the satellite. The returning radio waves are processed to produce a visual representation of the terrain or terrestrial-based object. Active imaging can also be used for mapping deep space and other LEO objects. Unlike the limitations of passive systems, active imaging can see through cloud cover and other obstructions and can operate day or night, in any weather condition. Moreover, active imaging can produce higher resolution images of the terrain or objects of interest, e.g., 1-4 meters in resolution.
In the present state of technology, most if not all satellites involved in active imaging are specifically designed and dedicated for that purpose. One LEO satellite may cover say 25% of the Earth's surface. Accordingly, four or more satellites are require for complete global coverage. Active type satellites are particularly useful for active imaging applications because of the improvement to image quality over passive systems. The cost of deploying and maintaining multiple satellites dedicated to active imaging is high. There are only a limited number of active imaging satellites presently in operation due, in part, to economic constraints.
A need exist to expand the use of active imaging for terrestrial mapping and object identification.